The overall purpose of this study is to optimize whole testes transplantation technology to study Ret-mediated spermatogenesis. This technology has been used to mature testicular germ cells into spermatozoa, which have the ability to cause fertilization by intracytoplasmic sperm injection into species-specific oocytes. However, transplant efficiency of the donor testicle is not perfect as not all immature germ cells in the seminiferous tubules display complete maturation to spermatozoa. The specific aims of this proposal are to first optimize donor graft efficiency of whole testes transplantation technology and then to use it in a novel application to study important genes involved in postnatal testicular germ cell maturation. Ret is a tyrosine kinase receptor that utilizes a coreceptor and ligand to mediate downstream cellular responses including cell survival, proliferation, and differentiation. This gene is critical for postnatal germ cell maturation; however, the available deficient mouse models have been limited in studying postnatal events such as spermatogenesis, due to perinatal lethality. Whole testes tissue transplant technology will provide an opportunity to use these mouse models to study postnatal spermatogenesis for the first time. The optimization of this technique is important to achieve not only as a tool for our planned studies involving Ret, but for its potential impact on clinical and other research applications. If germ cell maturation can be conducted at a high efficiency rate, this technique will more readily be applicable to human germ cell maturation protocols involving cryopreservation of immature, prepubertal testes tissue prior to gonadotoxic chemotherapy. The clinical impact of this technology will be tremendous, as it will provide a pre-treatment option presently not available for pediatric male cancer patients who may become permanently sterile. Determining how Ret affects testicular function will add to our understanding of human male infertility and may provide an avenue for new treatment strategies for infertile men with nonobstructive azoospermia. [unreadable] [unreadable]